A diesel engine is provided with a diesel pump to supply fuel of high pressure. Conventionally, one diesel pump has been provided to each cylinder of the engine. However, recently, a common rail system is becoming popular in order to meet requirements for higher pressure. The common rail system is configured to supply fuel to plural cylinders from a diesel pump via a common rail.
Conventionally, fuel pressure for a commercial diesel engine has been equal to or lower than 1,000 bar (i.e., 100 MPa). In this case, a diesel pump of an inner plunger type has been popularly used. With this type of diesel pump, fuel is pressurized by driving a plunger toward the pump center.
Meanwhile, for a diesel engine for a recent passenger car, the fuel pressure is equal to or higher than 1400 bar (i.e., 140 MPa) as reaching 2,000 bar (i.e., 200 MPa). The fuel pressure is expected to be further increased. For such a diesel engine, a diesel pump of an outer plunger type is adopted. In this case, plural plungers are radially arranged and fuel is pressurized as the plungers are pressed outward. In general, the number of the plungers is two or three.
A diesel pump of a conventional outer plunger type has been disclosed in Japanese Patent Laid-open 2003-49745, for example. In the document, a housing has a divided structure constituted with a housing main body and a cylinder head. The housing main body is made of aluminum and the cylinder head is made of steel. The cylinder head is fastened with bolts to the housing main body. Cylinder portion forms a single body with the cylinder head. The cylinder portion is protruded toward the housing main body and is located within an opening of the housing main body. A plunger is inserted into the cylinder portion and the plunger is driven by a drive mechanism of the housing main body.
When the plunger reciprocates within the cylinder as being driven by the drive mechanism, high pressure is repeatedly applied to the cylinder. In order to obtain sufficient durability for high pressure of fuel, the conventional diesel pump has the cylinder head made of steel as described above.
Here, in the conventional diesel pump, there may be a case that segregation of a trace ingredient of sulfur and the like for cylinder material appears at the inner face of the cylinder. Such segregation could become a crack source causing reduced durability of the diesel pump. Since the segregation appears in random manner, it is difficult to totally get rid of segregation at a machined surface of the cylinder.
Particularly, the fuel pressure is increasing recently, and is reaching 2,000 bar (i.e., 200 MPa) as described above. Decrease in durability due to the segregation becomes a problem in order to manufacture a diesel pump for such a high pressure in which very high reliability is expected.
One way to avoid disadvantages caused by such segregation is to use special alloy steel containing less amount of impurity, such as sulfur. However, such special alloy steel is expensive and is poor in machinability. A conventional cylinder head is large in size and has many areas to be machined. Therefore, in the case that special alloy steel as described above is utilized for the cylinder head, productivity is lost and manufacturing cost is seriously increased.